The correct answer is: A. Myofibrils contain thick and thin filaments.
Skeletal muscle cells are called muscle fibers because of their long and cylindrical shape. The plasma membrane of muscle fibers is sarcolemma, the cytoplasm is sarcoplasm, and the smooth endoplasmic reticulum sarcoplasmic reticulum (SR) Sarcoplasmic reticulum is specialized becaused it is involved in the process of contraction, so it stores, releases, and retrieves calcium ions.
The functional unit of a skeletal muscle fiber is the sarcomere which consists of highly organized contractile myofilaments actin that are thin filaments and myosin (thick filaments), together with other support proteins.
Small, highly concentrated cell organelles that produce proteins are called <span>ribosomes. Ribosomes can be free-floating or attached to endoplasmic reticulum.</span>
